| Corrosion is the gradual chemical or electrochemical attack of a metal or alloy by agents in the surrounding atmosphere. The resulting reaction causes disintegration of the surface and material loss due to either the conversion of the component metal or alloy into a less adherent material such as an oxide or sulfide, or the dissolution of the material into the environment itself. In many cases, wear mechanisms may also be present and exacerbate the subsequent material loss. Corrosive attack takes many forms Praxair Surface Technologies supplies hundreds of coatings to protect against various types of corrosive attack, including:
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| chloride ions), or when an electrical current is passed between metals while joined underwater. One metal or alloy becomes the anode and corrodes at an accelerated rate, while the more noble metal or alloy dissolves much more slowly or even ceases to corrode. | |
| » | Chemical attack corrosion: If the pH of the environment is very low or very high, substrates can dissolve. |
| » | Hot corrosion: At very high temperatures (1100° to 1700°F, or 593° to 927°C), hot corrosion can occur on metal surfaces exposed to gases containing chloride or sulfur compounds. Molten sulfates condense and dissolve protective oxides from the metal, and then attack the metal directly. |
| » | Fretting corrosion: When hard surfaces rub against one another in a corrosive environment, the chaffing action scrapes away any soft, friable corrosion products that form on the contact surfaces. Because new metal is always being exposed to the corrosive medium, the metal loss is accelerated. |
| » | Cyclic corrosion: When metal surfaces are wet with corrosive fluids, then dried and re-wet, the concentration of ions in the droplet is very high and the solution is extremely corrosive. Metal parts that are exposed to such heating and cooling in the presence of moisture suffer aggressive, accelerated corrosion. |
| » | Aqueous corrosion: When dealing with destructive service chemistries (such as sulfuric, hydrochloric, nitric, and hydrofluoric acids), aggressive corrosion can occur on the metal substrate. |
Without protective coatings, corrosion adversely affects the performance and durability of components' structural materials. For example:
| » | Gas turbines: In the power generation and aviation industries, corrosion can lead to pitting that can compromise the mechanical integrity of rotating compressor blades and vanes. Corrosion and oxidation also roughen the gas-washed surface of an airfoil, increasing aerodynamic drag and reducing compressor efficiency. |
| » | Reaction vessels and tanks: In the chemicals industry, highly corrosive liquids such as nitric, sulfuric, and hydrofluoric acids attack metal tanks and piping. |
| » | Risers: In the petroleum industry, large steel legs that support offshore drilling platforms stand in salt water that corrodes and weakens unprotected steel. |
Coatings are one of the most powerful tools available for limiting or modifying the response of an engineered component to its corrosive operating environment. Praxair offers an unmatched variety of coatings and treatments to protect surfaces against corrosion, several of which have become industry standards, such as:
SermeTel® metallic-ceramic coatings
| » | SermeTel® W aluminum-filled ceramic (inorganic phosphate) coating is galvanically sacrificial on steel and is stable in continuous use up to 565°C (1050°F). The SermeTel W coating's unique combination of heat and corrosion resistance was key to enabling the use of steel in flight turbine engines in the 1960s. The coating is in widespread use, specified by OEMs for protection of cases, shafts, and other components in flight and ground turbines. |
| » | SermeTel® 72 sealed, aluminum-filled, corrosion-resistant coating is especially resistant to corrosion in high-salt environments, and has set the standard for protection of compressor cases, wheels, and disks for several decades. |
| » | SermeTel® 5380DP aerodynamically smooth, sacrificial, corrosion-resistant coating is the most widely used coating to protect gas path surfaces on compressor blades and vanes. The development of smooth coatings proved essential to improving the efficiency and longevity of steel compressor cases and airfoils in turbine engines of all kinds. Consequently, this coating is widely specified by engine OEMs. |
| » | SermaLon® fouling-resistant coating has been demonstrated to limit corrosion and fouling in compressors and steam turbines. Consequently, the American Petroleum Institute has described its use in API STD 617. |
| » | Thermal sprayed aluminum (TSA) is used to provide anodic corrosion protection on large steel structures, especially those operating in salt water (such as offshore drilling platforms). Not only oil companies have specified TSA for use on steel; TSA also has been used to protect steel bridges in New Jersey and Texas. |
| » | SermaShield™ coatings and linings that are 40 to 100 mils (1000 to 2500 µm) thick compete directly with tantalum, titanium, and C-276 metal substrates and glass-lined steel vessels in severe chemistry service. These linings resist concentrated acids (sulfuric, hydrofluoric, etc.) and other chemical solutions under full vacuum and pressures across the entire pH range. Because fluoropolymers are “non-stick” resins—the polymers are widely used to coat “non-stick” cookware and bakeware—Praxair developed proprietary systems to bond these chemically resistant coatings to the insides of large tanks and reaction vessels. We developed primers for thinner coatings and use a proprietary bonded wire mesh, which can be exotic alloy mesh depending on the chemistry, to anchor thicker linings. The interface of wire bonded linings even can be vented to exhaust destructive vapors that permeate the polymer lining. |
Praxair will work with you to determine the optimal corrosion-resistant coating for your application. We evaluate your part's function, dimensions, weight, alloy, and temperature exposure to select or develop the coating to address your corrosion prevention needs.
Table 1. Desired corrosion-resistance characteristics and associated Praxair Surface Technologies coating solutions and application processes
| Desired characteristics | | Coating family | | Commonly used application processes |
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| Protection from galvanic corrosion up to 1000°F (540°C) | | SermeTel® slurries | | Air-atomized spray and bake |
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| Protection against aggressive low-temperature hot corrosion | | Aluminide Silicon-aluminide | | Pack diffusion Slurry diffusion |
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| Protection against hot corrosion | | Cobalt-based MCrAly alloys Silicon-aluminide | | Low-pressure plasma spray (LPPS) Slurry diffusion |
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| Improved corrosion and galling resistance | | Co-Cr alloys | | High-velocity oxy fuel (HVOF) |
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| Environmental barrier to mitigate corrosion, by providing cathodic protection | | Zinc-aluminum | | Flame spray Arc wire spray (TSA) |
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| Prevention of corrosion, oxidation, and wear | | Abrasive-filled alloys | | Tribomet™ electrodeposition |
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